Many photo
software editing programs
give you certain options when saving JPEG (or JPG) images.
You can choose the "quality" or "compression" amount of
the image as you save it. But what does this really mean?
The JPEG image format is a "lossy" image format.
In other words, saving in the JPEG (JPG) format always attempts
to
take image information out of the image. Image quality
and resolution are never improved using JPEG. Even
saving using the highest level (least compression) will still
drop
information from the photo, though the lost
information will be in the brightest or least visible areas
the human eye cannot see anyway. Opening a JPEG and resaving
a JPEG will accumulate loss in the
photo.
JPEG compression works by examining pixel brightness
on a scale of
-1024 to +1024. The compression (or optimization)
works according to brightness - not by color.
Compression is greater in brighter areas of the image (where
changes are less visible) and less in darker
areas. JPEG optimization uses a complicated mathematical
formula that operates on 8x8 blocks of pixels.
These two images look almost if not completely identical. But
there is a larger difference only visible when resized
(recall
from our November
newsletter the
difference between resizing and resampling - click
here to see that newsletter). The JPEG image has much less
information. Below, the TIFF image on the left remains printable
after resizing, whereas the image on the right shows the blocks
of 8x8
pixels. JPEG optimization has
taken information out, which becomes visible when
resized 300%:
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Resized TIFF 300% |
Resized JPEG 300%
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Why is this important? Many people think that JPEG images
- because they often look very good on web pages
or in photo directories are good enough quality for printing.
Remember though that JPEG was developed for use on the
web, where
file size considerations are very important. Before the
web, graphic artists had little use for GIF or JPEG images
because
they needed high quality images for printing on paper media
- not for computer screen presentation. So, when choosing
which format to save your images to, consider
what you are going to use the image for - if
it is for printing, GIF or JPEG formats should be avoided,
and TIFF or possibly PCX should be considered as better (less
lossy) alternatives.
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